Thermal, compositional, and compressional demagnetization of cementite

The 1 bar Curie temperature, T-C, at which cementite (anthropogenic form of the mineral cohenite, nominally Fe3C) abruptly loses ferromagnetism, is found to be sensitive to small deviations from the stoichiometric cementite composition. Stoichiometric Fe3C begins to lose magnetic susceptibility at 187 degrees C. The T-C of ferromagnetic loss in cementite falls by about 13-14 degrees C, in either compositional direction, to the limits at either Fe-saturation or graphite-saturation. Formation of C vacancies in, or C stuffings into, Fe3C produces non-stoichiometry, disrupts and weakens the Fe magnetic ordering, and produces excess configurational entropy that is proportional to the disruption magnitude. C-excess (similar to 0.6 at% C) at graphite-saturation is less than the C-deficiency at Fe-saturation (similar to 2.6 at% C), so the rate at which Curie T-C drops with cementite C% variation is asymmetric about the stoichiometric composition, being steeper on the C-excess side. This asymmetry reflects the higher excess configurational entropy (and consequently greater weakening of Fe magnetic ordering) generated by C excesses than by C vacancies.